Is it possible to include a date with the <previously-shown> tags in the xml file? So, instead of:

<previously-shown>

Have this:

<previously-shown start="20161127000000" />

Omitting the date causes problems for a program I'm using (xteve, which integrates IPTV with Plex). I inquired with xteve's developer, and he felt it was a provider issue because of the missing date (although looking at the xmltv website, it *should* be valid without the date).

well this would depend on the situation.
the grabbing software is setup so that the date can be added if its present.
but...
many times the previous shown element is obtained from something as simple as the word "Repeat" in the data so no date is available and the result is the tag in your example.